The present invention relates to a method for recognizing properties of a material that are of interest, based on the volatile and/or vaporizable portions of this material. The invention is used in particular to be able to make comparisons of materials/substances basically considered identical/similar, in order to recognize whether there are actually differences between these compared substances with respect to the concerned properties. A specific field of application is the food sector, for example, in order to be able to make quality categorizations of batches thereof.
It is known that in organic chemical analysis, mass spectrometry is the most important detection method for identifying unknown individual substances.
In particular the combination of mass spectrometric detection with static headspace gas chromatography is a well-established routine analysis method for qualitative and quantitative determining of the volatile components of an unknown sample. This method of analysis is based on the fact that in a closed container, the very volatile and moderately volatile components of a sample spread out between the sample matrix and the gaseous phase over this matrix. In this connection, the sample matrix itself may be liquid or solid. In the state of thermodynamic equilibrium, the gaseous phase contains a qualitatively and quantitatively representative equivalent of all volatile components. For this reason, for identifying the volatile components of an unknown sample, a defined gas volume, what is referred to as the aliquot, is removed from the gaseous phase and is fed to the gas chromatographic separation with subsequent mass spectrometric detection.
It is known to use static headspace gas chromatography in combination with mass spectrometry in different areas of organic chemical analysis.
It is known from Ragunathan, N. et al. xe2x80x9cMultispectral detection for gas chromatographyxe2x80x9d JOURNAL OF CHROMATOGRAPHY A, vol. 703, no. 1, 26th May 1995, pp. 335-382 XPOO4023371, to timewise separate an unknown mixture of substances into its single components, which can be sequentially analyzed by a mass spectrometer. For each single component an individual mass spectrum can be recorded within a respective time window. In the following spectral analysis the individual mass spectrum of each single component is compared to a given catalogue (library) containing the mass spectra of various reference substances. To reduce library search time the system selects only a few ionized fragments from the individual mass spectrum to be compared with the reference spectra by introducing a threshold for the minimum peak height. The intensity of the spectral line determines which spectral lines are used for the spectral comparison. The system selects only a reduced number of spectral lines for evaluation.
From Belitz and Grosch, xe2x80x9cLehrbuch der Lebensmittelchemiexe2x80x9d [Textbook of Food Chemistry] (1992), p. 312, it is known that this method of analysis, known under the acronym HSGC/MS, is well-established in particular in the food industry sector, because the type as well as the number of volatile components contained in a food sample are of decisive significance for the quality of a food. In particular the presence of certain odor-active volatile components that mark the aroma of a food sample, and their concentration in the food sample, are used in this connection as the basis for quality assessment.
Likewise, in numerous sectors of environmental analysis, in particular in utilization of waste products, in environmental monitoring, in emission studies of packaging material and in emission analysis, the routine decoding of the volatile components of a sample to be examined takes place by means of the HSGC/MS method. In this case as well, the odor-active volatile components are particularly significant because they are used to assess the annoyance of emissions. HSGC/MS analysis has similar importance for determining maximum workplace concentration values.
From Newman, xe2x80x9cElectronic Nosesxe2x80x9d, Analytical Chemistry 63 (10), pp. 585A-588A, it is known that in recent times, in particular in the field of food quality control, instruments that are referred to as xe2x80x9celectronic nosesxe2x80x9d are used for the objective, rapid measuring/characterization of odors. These are sensor arrays of several unselective individual sensors that are housed together in a measuring chamber. Just as in HSGC/MS analysis, the xe2x80x9csamplexe2x80x9d arriving for measuring is an aliquot of the gaseous phase that is situated above the actual liquid or solid sample matrix. The individual signals delivered by the individual sensor elements when volatile substances are present in the gaseous phase are evaluated by methods of pattern recognition, known from Gardner and Bartlett, Sensors and Sensory Systems for an Electronic Nose (1992), p. 161. In this connection, cluster analysis in the multidimensional space or neural networks are preferably used.
In particular when studying odorous samples with the help of sensor arrays, there is no identification of the volatile components contained in the sample. For this reason, neither the components decisive for the odor of the sample nor the odorless components become known thereby. If two odorous samples differ only in that non-odor-active components are present in different concentrations, both samples are classified as different by the xe2x80x9celectronic nosexe2x80x9d, although they are assessed identically in human sensory perception. Such an erroneous classification limits the ability of xe2x80x9celectronic nosesxe2x80x9d to be used universally.
In practical experience, in particular in the food sector, the technical problem that is often presented is to recognize materials, e.g. from batches, goods deliveries and the like in order, for example, to be able to carry out a quality classification thereof, or to allow evaluation comparisons between substances considered identical/similar, where this supposed parity actually does not hold true. A simple example is provided not for limitation, but rather only as an illustration. Different batches, e.g., of parsley delivered should actually be of uniform quality. For example, these batches cannot be differentiated optically from each other, yet they display odor differences. The recognition to be executed with the invention is intended to allow objective differentiation of the individual batches, for example for the purpose of classification into merchandise categories.
A technical problem addressed by of the present invention is to indicate a method that makes such recognition or differentiation executable at the least possible expense and in particular in a brief time.
This technical problem is solved with the teachings of patent claim 1, and in further developments according to the subclaims.
The invention is based on basically known recording and evaluation of mass spectra of complex substance mixtures, but including, according to the invention, measures that allow mass spectrometry, familiar in principle, to be applied in a simplified manner in that a given (random) reference sample is taken as a basic pattern, and the actual series of tests are conducted only as comparisons with simplified expenditure. In particular, when examining and evaluating the individual (series) samples of the testing series, the method according to the invention does so without conducting gas chromatographic separation of the components of these samples in each case, namely without loss of quality or general validity of the (quality) assessment achieved.
The method according to the invention has the advantage that in (merchandise) classification not every individual sample is to be examined in detail in a time-consuming manner. Rather, as is also shown more thoroughly in the following detailed description, only a one-time calibration procedure is carried out with a (random) reference sample for this merchandise, namely preferably with an arrangement also to be used for conducting the subsequent series tests, as is described in FIG. 2, for example.
With respect to a pattern analysis/evaluation used with the invention, the following should be noted:
A pattern (of a sample) relevant in this case comprises a number n of pattern values which, taken together, are to be considered an n-dimensional vector in the n-dimensional space and form the pattern. Two patterns concurring signifies that the vector of one pattern and the vector of the other pattern concur in length and direction. For the practical execution of pattern analysis, however, it is necessary to take into account/predetermine a tolerance space depending on the technical problem/requirement for such a concurrence. That means that the concurrence of two or more patterns already exists when the tip of the vector of the given pattern is situated within this n-dimensional tolerance space (provided that the vectors have a concurring point of origin).
An individual pattern with n pattern values is obtained when one determines from a (first) reference sample the number n of predetermined characteristic values thereof. If one examines a seemingly identical second reference sample which, as is typical in practical reality, is only nearly identical to the first reference sample, a second, only nearly identical pattern is obtained. Several such reference patterns taken together lead to or form in this case (according to a first variant for execution of the invention) a reference random sample for which there accordingly results a random sample pattern of values objectively determined and/or taking tolerances into account; this random sample pattern is to be considered a vector with the tolerance range of its length and its direction (=tolerance space). This is synonymous with the term (reference) random sample pattern for this variant. A second variant is to determine and/or set such a tolerance space for the reference random samples e.g. from experience (experimental values), derived from requirements of the technical problem and the like. For this, only one reference sample is then needed for the pattern vector as the one to which, for the characteristic of serving as reference sample pattern, the predetermined tolerance is thus (subjectively) assigned.
Here it should be briefly noted that according to the invention with (at least) one selected reference sample and one reference random sample, this is formed (according to the first variant) from a representative selection of reference samples in statistically sufficient number or (according to the second variant) with a predetermined tolerance space, a testing measure belonging to the invention is conducted in two phases, in order to create, for this reference random sample with its tolerance space of its pattern vector and for the series tests e.g. of the merchandise batches to be conducted subsequently, a pattern, as also described in more detail, adapted to the technical problem and with only a limited number of ionized fragments of the volatile components of the reference random sample.
In the first phase of this testing measure, from (at least) one reference sample the mass spectra of the individual substance components (of interest) are determined separately from each other by gas chromatography. In this connection, each such individual mass spectrum of a single substance component contained in the reference sample consists of 50 to 250 ionized fragments, for example.
It is basically sufficient in this first phase to record, as indicated, the individual mass spectrum of only a single selected reference sample to save time and money. To completely rule out the possibility that a perhaps less pertinent selection of this single reference sample could lead to a perhaps less favorable result to be achieved with the invention, it may be useful to record in this first phase the respective individual mass specta of several selected reference samples (required in the subsequent second phase for the above first variant).
For the practical reality of a pattern recognition analysis solving the technical problem, such a number of ionized fragments of the individual components of the sample is already too great to take them all into consideration. For this reason, the invention provides, by mass spectrometry in the second phase of the testing measure, for continued detection and evaluation as pattern values of only a reduced number (described below) of selected ionized fragments, e.g. 10, namely of the overall mass spectrum of the unseparated reference sample (in which all individual parts of the sample are thus contained). This reduced number of ionized fragments/pattern values then also serves as the basis for the pattern recognition analysis of thexe2x80x94according to the technical problem of the present inventionxe2x80x94brief series testing of batches/series samples that are to be compared in each case with the pattern or (as in this case for two variants) with the reference random sample pattern defined.
One selects form these individual mass spectra of the first phase those ionized fragments that occur particularly specifically and/or quite dominantly in the respective individual mass spectrum of volatile content substance/substance component and/or are an indication/characteristic for the particular property /quality of the entire reference random sample to be formed. According to the second phase, a reduced mass spectrum is taken of each undivided reference sample of the reference random sample, which consists of only the 20 ionized fragments, for example (determined in the described manner). Thus, no gas chromatography breakdown of the reference samples is carried out in this second phase. Depending on the property/quality that is to be recognized, a weighting of the relevant ionized fragments in the pattern recognition process can then still be carried out.
The above-described recommendation to record the individual mass spectra of more than just one reference sample then apparently serves to obtain a particularly optimally pertinent selection of the ionized fragments uses in reduced number for the further pattern comparison.
The following comparison of the reduced overall mass spectra determined in each case by the individual, unseparated samples of the batches of the series test with the pattern of the reference random sample is carried out by means of a pattern analysis based on the weighted, selected ionized fragments, e.g. 20 such weighted ionized fragments are evaluated.
This summary shows that the actually time-consuming phase of mass spectrometric examination with gas chromatographic breakdown into portions of the content substances of a sample only has to be carried out once and (usually) only for one reference sample.
A further simplification for a further development of the invention results, for example, from drawing up catalogues of the patterns, consisting of the weighted reduced mass spectra of the reference random samples for various substances, e.g. various types of vegetable, fruit and the like; these catalogues are used repeatedly when new goods are delivered in order for the comparison to be made according to the invention. In this connection, as described above, for the samples of the series tests in each case only the respective reduced overall mass spectrum of this merchandise sample then needs to be recorded and evaluated, namely without gas chromatographic breakdown of the merchandise samples.